[FEAT] Add floating "Scroll to Top" button on Dashboard

[Rashi1404]

Problem Statement

As a user, I want a quick way to return to the top of the Dashboard after scrolling through multiple widgets (commit chart, PR analytics, goals, streak tracker, etc.) so that I can navigate efficiently without manual scrolling.

Proposed Solution

• Add a floating button fixed to the bottom-right corner of the Dashboard
• Button appears only when user scrolls past 400px
• Clicking it smoothly scrolls to top using window.scrollTo({ top: 0, behavior: 'smooth' })
• Use Lucide ArrowUp icon
• Style with Tailwind: fixed bottom-6 right-6 p-3 rounded-full bg-primary text-white shadow-lg hover:scale-105 transition
• Respect dark/light theme

Feature Area

Dashboard

Alternatives Considered

• Browser-native scroll (poor UX on long dashboards)
• Sticky header with back-to-top link (takes up screen space)

Acceptance Criteria

[ ] Button is hidden when at top of page
[ ] Button appears after scrolling 400px down
[ ] Clicking button smoothly scrolls to top
[ ] Button is visible in both light and dark mode
[ ] Button does not overlap with other floating elements (e.g., Discord webhook toast)

Additional Context

No response

[Honey-pg]

/assign me.

[github-actions]

Assigned to @Honey-pg! You have 7 days to open a PR. No PR after 7 days = auto-unassigned so others can pick it up. Good luck!